Vinyl aromatic/allylic alcohol copolymers are well known (see U.S. Pat. Nos. 2,630,430, 2,894,938, and 2,940,946). Copolymers of styrene and allyl alcohol (SAA copolymers) are resinous polyols that are particularly useful for polyesters, fatty ester emulsions, alkyd and uralkyd coatings, melamines, and polyurethanes. Styrene-allyl alcohol copolymers can be made in a batch process by charging a reactor with styrene, allyl alcohol, and a free-radical initiator, and heating the mixture at a temperature effective to polymerize the monomers (usually 100.degree.-300.degree. C.).
While SAA copolymers having relatively low hydroxyl contents (hydroxyl number=50 to 150 mg KOH/g) are known (see, e.g., U.S. Pat. No. 2,894,938, Example 1), the utility of these polymers was previously thought to be rather limited (see U.S. Pat. Nos. 5,444,141 and 2,940,946). Those skilled in the art believed that a higher concentration of SAA hydroxyl groups was needed for sufficient crosslinking and good coating properties.
We recently described a process for making vinyl aromatic/allylic alcohol copolymers (U.S. Pat. No. 5,444,141). The process is well-suited for making SAA copolymers having hydroxyl numbers within the range of about 180 to 280 mg KOH/g, the range generally recognized as commercially valuable. Commercially available resins include "SAA 100" (hydroxyl number=200 mg KOH/g) and "SAA 101" (hydroxyl number=260 mg KOH/g) resinous polyols. We demonstrated that yields of these SAA copolymers improve significantly when the free-radical initiator is gradually added to the reaction mixture. We reported polymer yields of 30-40% for products having hydroxyl numbers of 180-280 mg KOH/g.
In U.S. Pat. No. 5,444,141, we said that the yield of SAA copolymer can be increased, "but only at the expense of making a product having higher styrene content, lower hydroxyl number, etc., a product that lacks utility for most of the targeted end-use applications."
For some coating applications, however, hydroxy-functional resins with lower hydroxyl numbers are actually an advantage. Some coating components are incompatible with SAA copolymers that have a high content of hydroxyl groups. Low hydroxyl content can also be an advantage in reducing the amount of relatively expensive crosslinking agent (e.g., polyisocyanate) required.
In sum, an improved process for making vinyl aromatic/allylic alcohol copolymers, especially styrene-allyl alcohol copolymers, is needed. Coating applications that could benefit from a low hydroxyl group content resin are evolving, and suitable resins are needed for these applications. A process for making the resins is therefore also of interest. Preferably, the process would be easy to perform, would use conventional equipment, and would be cost-effective. A preferred process would give favorable yields of SAA copolymers having hydroxyl numbers within the 50 to 150 mg KOH/g range.